Roles of TSP1 and PICK1 in neuroligin-mediated synaptogenesis

Synapse formation and elimination in the central nervous system play critical roles in establishing neural networks and building a foundation for complex brain tasks, such as learning and memory. The proteins neurexin and neuroligin are located at opposite sites on the synaptic membrane and directly interacts with each other to generate new synapses. The extracellular matrix protein thrombospondin (TSP) is also reported to induce synapse formation through some unknown receptors. Interestingly, these neurexin-neuroligin or TSP induced synapses all lack AMPA type glutamate receptors and consequently, synaptic responses. Protein interacting with C kinase 1 (PICK1) interacts with both the AMPA receptor subunit GluR2 and neuroligin. Recent studies also show that PICK1 maintains a synaptic pool of AMPA receptors and regulates their surface expression. Here, we report that during early development, TSP1 accelerates the speed of the functionally silent synapse formation without AMPA receptors. We identified neuroligin 1 (NL1) as one of the TSP receptors in inducing synaptogenesis. When the extracellular domain of NL1 was applied in the culture medium, or when endogenous NL1 was knocked down in hippocampal neurons, the synaptogenic effect of TSP1 was blocked. This defect can be rescued by overexpressing NL1. We also found that synaptic targeting of PICK1 would subsequently recruit AMPA receptors onto neurexin-neuroligin induced synapses and thereby facilitate synapse maturation. This synaptic recruitment of AMPA receptors depends on the direct interaction between GluR2 and PICK1, as well as PICK1’s lipid binding ability. Moreover, we show that one of PICK1’s interacting partners ICA69 is involved in this GluR2 trafficking process and serves as a negative regulator. Our results provide a new understanding of how neuroligin is involved in early synapse formation and how neuroligin-induced synapses mature.